Sometime ago I saw dry ice pellet blasting/cryo pellet blasting used on fully built operation vehicles. The pellets would take off the tar, grease and gunk yet leave the paint/zinc/coatings alone as would ~pulverize upon impact. Basically it was sandblasting, on a very safe scale as was just enough to pull dirt off. The problem? It's only provided by 3rd parties who drive a ~large machine to your home.
Per the fender above in the before pics, you can see the brass bristle scratches trying to get through the gunk after 15min of soaking in Purple Power Cleaner...what you see is what I really couldn't get through with a brush (raw metal showing in after pic is tire burn).
$3.96 10lb bag, 90 psi, didn't even use the whole bag. Within 10 seconds you're greated by the EXACT smell of a cotton candy machine. The results happened with out even trying.